The single phase or multi-phase generator that constitutes a conventional alternator for a motor vehicle generally comprises a stator within which a rotor, which carries an excitation winding, rotates. The excitation winding is energized through brushes in contact with two slip rings arranged on a projecting portion of the shaft of the rotor.
Various rotary machines are known, in particular from European patent specification No. EP 0 707 374, in which the windings of the stator are excited at the same time by permanent magnets and by excitation windings, and in which the current delivered by the armature to the excitation windings is controlled by switching, with inversion of the excitation current serving to reduce the flux of the magnets at high speeds.
However, since the excitation current is, by nature in this type of machine, a two-way alternating current, such switching makes it necessary to have recourse to an H-shaped switching bridge, which is an expensive item.